658 



can be connected to a space of constant pressure and a barometer 

 through tlie stopcock k^. The ground joint / may be used for ex- 

 haustion, for drying with the aid of a moisture-catcher placed in 

 liquid air and for tilling. 



The oxygen was prepared from potassium-permanganate in an 

 apparatus which could be evacuated by means of a Toepler-pump 

 and dried by freezing with liquid air. The gas was condensed in a 

 small bulb which was attached to the apparatus and cooled in liquid 

 air, whence it was conveyed to the vapour-pressui-e apparatus by 

 means of the air-pump ^). 



The nitrogen was prepared by Dr. Lemkes under supervision of 

 Pi'of. VAN Itau.ik, to whom I express my sincere thanks. 



1) One third of the distillate wa? used for rinsing out the apparatus, the second 

 third for the filling. 



') In order to prevent possible misunderstanding it is mentioned here that the 

 pressure is given in "'international centimeters" i.e. in the 76*^ part of an inter- 

 national atmosphere or in other words as the height of a mercury-column measured 

 not at Leiden itself (local cm.), but at a place where one international atmosphere 

 exactly agrees with 76 cms mercury at 0° C, i.e. where gf = ^Bur. int. : 1-0003322, 

 the value whicli is taken as g'nonn. and at present is to be put at 980.615. (At 

 Leiden g = 981.276 and 1 int. atm. = 75.9488 cms). For the definition of inter- 

 national atmosphere Comp. Leiden Comm. Suppl. 23. Mathem. Enc. V 10. Einh. a. 

 When in these Communications about observations at Leiden an atmosphere pure 

 and simple is mentioned, the international atmosphere is always meant. 



